1. Field of the Invention
This invention relates to a novel method of preparing indicator dyes, to novel intermediates useful in the preparation of the dyes and to the preparation of the intermediates.
2. Description of the Prior Art
Dyes which undergo a change in spectral absorption characteristics in response to a change in pH are well known in the art and are frequently referred to as indicator or pH-sensitive dyes. Typically, these dyes change from one color to another, from colored to colorless or from colorless to colored on the passage from acidity to alkalinity or the reverse and are commonly employed in analytical chemical procedures to measure changes in pH value. Among the indicator dyes most widely used is the group derived from phthaleins as exemplified by phenolphthalein, thymolphthalein, o-cresolphthalein and 1-naphtholphthalein.
Various methods are known in the art for preparing phthalein indicator dyes. In one of the more conventional procedures, phenols, such as thymol, o-cresol, and phenol itself are reacted with phthalic anhydride at elevated temperatures in the presence of a suitable catalyst such as zinc chloride or sulfuric acid to yield the corresponding symmetrical, i.e., di-phenol phthalein. Di-indole phthaleins also have been prepared by simple condensation usually in the presence of an acid catalyst and by other methods, such as, reacting magnesium indyl bromide with phthalyl chloride
Another method of synthesizing indole phthaleins is disclosed in British Pat. Nos. 1,160,940; 1,161,386; 1,161,387; and 1,162,771, which comprises reacting an indole with phthalic anhydride in the presence of a metal halide, e.g., aluminum chloride to yield a keto-acid intermediate which is substantially reacted with a second aromatic compound, the same or different, in the presence of an acid condensing agent to yield the desired indole phthalein. Using this method both symmetrical and unsymmetrical compounds may be prepared by selecting respectively, as the second aromatic compound, an indole which is the same or an indole which is different from the starting indole initially reacted with the anhydride. Where it is desired to produce a mixed indole phthalein containing an indole radical and a second radical derived from a different aromatic compound such as carbazole or aniline the keto-acid intermediate may be formed by reaction of the indole, carbazole or other appropriate compound with phthalic anhydride followed by condensation of the intermediate with the second aromatic compound to yield the desired mixed indole indicator dye.
These prior methods of preparing phthaleins, though useful in synthesizing a large number of compounds, are accompanied by certain drawbacks. The simple condensation reactions and the Grignard reactions are not useful with all starting materials. Some phenols and indoles will not react under the condensation conditions conventionally employed. Moreover, these synthetic methods generally are limited to the production of symmetrical compounds, i.e., di-phthalides containing two indole or two phenol radicals that are identical. While the method of the aforementioned British patents is useful for producing symmetrical and unsymmetrical indole phthaleins and also mixed indole indicator dyes, the more sensitive indole derivatives, when used as starting materials, tend to decompose under the vigorous reaction conditions encountered in the presence of the aluminum chloride catalyst.
It is known from the work of Brubaker, et. al., J. Amer. Chem. Soc., 49, 2279 (1927) that o-phthalaldehydic acid condenses with phenol and certain substituted phenols having a free para-position to yield the para-condensation products. These compounds were prepared by mixing equimolar proportions of phenol and phthalaldehydic acid and then adding a suitable condensing acid, such as sulfuric acid, while maintaining the reaction temperature below about 30.degree. C.
As reported by Norland, et. al., ibid., 82, 5143 (1960), phthalaldehydic acid and indoles will condense to yield phthalidylindoles and water when the two reactants are fused together at temperatures of 120.degree. to &gt; 200.degree. C. If the 3-position of the indole is free, then 3-phthalidylindoles are formed. If the 3-position is blocked or if the 1- and 3-positions are blocked, then 1-phthalidylindoles and 2-phthalidylindoles are formed, respectively.
Rees, et al., J. Chem. Soc., pp. 680-687 (1965) observed that for reaction with phthalaldehydic acid the 3-substituted indoles and the 1,3-disubstituted indoles generally require the vigorous fusion conditions used by Norland et al., but found that indoles having a free 3-position will react with phthalaldehydic acid under milder conditions. Indole and its 2-phenyl, 2-methyl, 7-methyl and 1,2-dimethyl derivatives were reported to react in hot benzene to yield the corresponding 3-phthalidyl indoles which results were attributed to intramolecular acid catalysis. In solution phthalaldehydic acid exists in the cyclic form, 3-hydroxyphthalide, which is in rapid equilibrium with the open-ring form, o-formylbenzoic acid. Presumably, the indole reacts with the aldehyde form and the carboxyl group ortho to the aldehyde group functions as an intramolecular acid catalyst. The authors observed that reactions with indole and 2-methyl indole also were catalyzed by an external acid catalyst, such as toluene-p-sulfonic acid, and also that a second mole of indole could be added to 3-phthalidylindole by opening the lactone ring with alkali and treating the resulting salt with a second mole of indole.
Rees et al. in further studies, ibid., pp. 687-91 (1965), reported that the condensation of phthalaldehydic acid could be extended to pyrroles and found that phthalaldehydic acid reacted with pyrrole and 2,5-dimethyl pyrrole in boiling benzene in the absence of an external catalyst to give high yields of 2-phthalidyl-pyrrole and 3-phthalidylpyrrole, respectively. In these reactions, it was observed that pyrrole tended to substitute in the 2-position when possible to yield the 2-phthalidyl derivative. In a further extension of the reactions, the authors found that naphthalaldehydic acid, though less reactive than phthalaldehydic acid, behaved in a similar manner and could be condensed with indole under fusion conditions to yield 3-naphthalidylindole.
It has now been found that certain phthalidyl-substituted phenols, naphthols, indoles and pyrroles and the corresponding naphthalidyl derivatives may be oxidized to yield novel intermediates which will condense readily with another mole of the same aromatic compound as used in the initial condensation reaction or a different aromatic compound, to yield a phthalide or naphthalide indicator dye.